This invention relates generally to a method and apparatus for measuring the thickness of a workpiece by the utilization of ultrasonic pulses, and more particularly to such a method and apparatus wherein zero-point adjustment can be accurately effected with a simple operation.
There have been known thickness measuring apparatuses wherein ultrasonic pulses are caused to be periodically transmitted into a workpiece from one surface thereof, and ultrasonic echo pulses reflected from the bottom surface of the workpiece are received so as to measure the thickness of the workpiece from the propagation time of the ultrasonic pulses through the workpiece. Such thickness measuring apparatuses generally employ a probe comprising a transducer provided at its front surface with a delay material in order to improve the separation between the transmitted pulses and the received pulses. In such apparatus, in order to obtain the propagation time of the ultrasonic pulse corresponding to the thickness of the workpiece, the process of subtracting the propagation time of the ultrasonic pulse through the delay material from the total period of time elapsed from the transmission of each ultrasonic pulse to the reception of its related ultrasonic echo pulse is required. Generally, the apparatus has a zero-point adjusting circuit included therein which is used to set the period of time corresponding to the propagation time of the ultrasonic pulse through the delay material. More particularly, the zero-point adjusting circuit provides a signal an adjustable period of time after the transmission of each ultrasonic pulse, such signal being applied to a gate adapted to be enabled from the time of reception of such signal to the time of reception of the related echo pulse reflected from the bottom surface of the workpiece to thereby develop a signal having a duration proportional to the thickness of the workpiece. The operation of adjusting the setting of the zero-point adjusting circuit is referred to as "zero-point adjustment operation".
Conventionally, such zero-point adjustment operation has been performed by causing the probe to contact a standard test-piece of a known thickness and adjusting the setting of the zero-point adjusting circuit until the thickness value displayed on the display reaches substantially the same value as the known thickness value.
However, the known thickness measuring apparatus has the defect that the zero-point adjustment operation is rather troublesome and time-consuming because it requires the use of a standard test-piece of a known thickness, the defect that a relatively long time is wasted because such troublesome and time-consuming zero-point adjustment operation must frequently be effected for the reason that the propagation speed of the ultrasonic pulses within the delay material varies depending upon the ambient temperature, and the defect that, particularly in the case of a small-size portable measuring apparatus having a test-piece attached on the outer casing thereof, the contact medium material to be used with such test-piece may flow out and spread to contaminate the apparatus or may flow into the interior of the apparatus through gaps between the casing thereof and the adjusting devices and meters provided therein, thus resulting in malfunction of the apparatus.